The present invention relates to an icebreaker.
In the case of ships having traditional icebreaking configurations with pointed bows, very high external forces are introduced horizontally into the ice surface by means of the hull plating particularly in the water line region, the wedge-shaped bow water lines and the front shoulders of the hull form, the ice surface being destroyed by compression and bending breaks. The high external forces, particularly in said critical plating zones, lead to very high friction force components, which are prejudicial to the bending breakage of the ice surface and the propulsion of the ship.
In the case of ships having a favourable icebreaking bow shape, break lines are produced in the ice in front of the ship and run at right angles to the longitudinal axis of the ship. Thus, ice floes are obtained, which initially have a width corresponding to that of the hull. These ice floes are forced under the hull, at a certain depth are broken in planned manner into two halves and are then led away to the side. In this process, the ice floe with its original surface is guided by the outer plating of the prow. At its leading edge the floe is supported against the unbroken ice. The supporting force acts on the unbroken ice in bottom to front sloping manner and its action direction is opposed with one component to the force intended to crush the ice in front of the ship into the aforementioned ship-wide floes and consequently reduces the breaking force. The ship must simultaneously apply the longitudinal component of this force through increased propeller or screw thrust. As the longitudinal component of the breaking force must also be consumed by the propeller thrust, the supporting of the broken floe and all floes behind it in the forwards direction has a double effect on the propeller thrust, in which all floes "stick" under the ship's bottom jointly have a double action, firstly due to the frictional force component opposing the thrust and secondly through the frictional force component opposing the breaking force. The first frictional force component consequently requires a higher propeller thrust, even if the friction is absorbed by surroundings other than the frontally positioned ice. The second component cancels out part of the breaking force, so that through a further increase in the thrust and consequently the ship must be moved even higher onto the ice than if no frictional force were present.
DE-OS 21 12 334 discloses a ship with an icebreaking bow, whose hull passes into an underwater prow with two wedge-shaped icebreaker stems forming a channel between them. At the rear end of the channel is provided a snowplough-like guide device beneath the ship's bottom. The resulting much smaller ice floes cannot be moved under the lateral, fixed ice cover and instead float in the gap between the hull and the lateral fixed ice cover and cause increased friction on the hull plating or collect in the channel and slide midships under the vessel into the propeller region. Thus, such a ship has an increased power requirement and the propellers are exposed to the damaging effects of the ice floes.
Furthermore, European patent 00 79 002 discloses the shape of a prow, e.g. for icebreakers. For travelling in open or ice-covered waters, the ship is provided with a pontoon-like prow located above the water line, which has parallel side walls and an end face extending over the entire width of the ship which, in the underwater part, is planer and slopes greatly forward. Towards the rear it passes into a centre keel and is provided with a stern and drive means housed in the latter. The construction is such that the lateral edges in the transition region between the prow side walls and the end face are curved in the longitudinal direction of the lateral edges and protrude laterally with respect to the planes formed by the prow side walls, so that the distance between the lateral edges located below the construction water line forms the maximum width of the underwater ship. The under sides of the frames between the two lateral edges from the point of the ship's length at which the end face passes into the centre keel and up to the point at which it reaches the ship's bottom are constructed athwartships in downwardly curved or bent manner. This construction is intended to provide a ship with a limited power requirement for propulsion without great technical, design expenditure and in particular with icebreaking characteristics. In addition, there are even more favourable conditions for the shear fracture of a one-piece floe from the fixed ice cover, accompanied by an improvement in the guidance of the floe under the water with reduced risk of crushing thereof into small pieces, so that the lateral transfer of the floe below the fixed ice cover is even more reliably achieved.
In all icebreakers, the support of the broken ice floe and the ice floes behind it in the forward direction has a multiple effect in the propeller thrust. This forward support would not be necessary if there were no frictional forces between the hull plating and the ice surface of the submerged ice floes. These frictional forces can never be completely prevented, but the magnitude thereof is decisively dependent on the contact between the ice surface and the outer plating. Vital importance is attached to the presence of a lubricating film between the outer plating and the ice. Even if no water can penetrate the space between the outer plating and the ice, in the case of a sufficiently large relative movement between ship and ice, the frictional heat in itself forms a lubricating film from the melted ice. In addition, the thermal state of this region is such that the melting heat is greater than the heat removed. However, this no longer applies in the case of slow ships movements, i.e. in the limit range of icebreaking by the ship and with very low outside temperatures. The small frictional heat produced at such a low speed is rapidly removed again by the very cold, surrounding ice and by the very cold steel of the hull plating, so that at the most there is dry friction leading to a very high frictional force. When movement is topped, it is even possible for the ice floe to freeze solid with the steel of the outer plating, so that it is very difficult to detach the said ice floe from the ship.